Severe, combined immunodeficient (SCID) mice also provide a model for evaluating anti-influenza treatments in an immunocompromised host. In a recent study, SCID mice infected with an influenza A virus and treated with either oral oseltamivir or the experimental compound A-322278 showed reduced viral replication, limited weight loss and prolonged survival, so long as the treatments persisted (Sidwell et al., 2003). However, once treatment was discontinued, the animals had detectable, progressive viral replication with subsequent clinical decline. More importantly, drug-resistant virus was detected in the chronically infected, drug-treated mice, but not in the placebo-treated animals that died rapidly from infection.